1 Field of the Invention
This invention relates to integrated circuit manufacturing and more particularly to determining the roughness of an unpatterned surface of a semiconductor topography.
2. Description of the Relevant Art
Fabrication of an integrated circuit is a complex process involving numerous steps. To form a metal-oxide-semiconductor (MOS) transistor, for example, a gate dielectric is formed on a semiconductor substrate which is doped with either n-type or p-type impurities. A gate conductor is formed over the gate dielectric, and dopant impurities are introduced into the substrate to form a source and drain. Such transistors are connected to each other and to terminals of the completed integrated circuit using conductive interconnect lines.
A pervasive trend in modern integrated circuit manufacture is to produce transistors having feature sizes as small as possible. Many modern day processes employ features, such as gate conductors and interconnects, which have less than 1.0 xcexcm critical dimension. As feature size decreases, the sizes of the resulting transistors as well as that of the interconnects between transistors also decrease. Fabrication of smaller transistors allows more transistors to be placed on a single monolithic substrate, thereby allowing relatively large circuit systems to be incorporated on a single, relatively small die area.
This trend toward reduced feature sizes imposes severe demands on the lithography processes used to define features in integrated circuit fabrication. In a lithography process, a film of a radiation-sensitive material called photoresist is typically formed upon the surface of the material to be patterned. This photoresist film is then exposed through a mask to radiation. Portions of the photoresist which are exposed to the radiation undergo a chemical change, such that subsequent use of a chemical called a developer will exclusively remove either the exposed or unexposed resist portions. In this way, the mask pattern may be transferred to the photoresist. The retained photoresist may then be used as a mask for subsequent etching or doping of the underlying material, thereby transferring the mask pattern to this material. Complications with photolithography processes can occur, however, which cause the feature sizes of the patterned material to be different than those of the mask. Such feature size differences become even more significant as feature size is reduced, and the xe2x80x9cerrorxe2x80x9d in the patterned dimension may approach the magnitude of the intended dimension itself.
One source of feature size error in photolithography processes may be scattering of exposing radiation from rough surfaces underlying a photoresist film. A situation in which this may occur is illustrated in FIG. 1. Photoresist film 2 is formed over material 4 which has a rough upper surface. Exposing radiation 6 is directed into photoresist film 2 through a transparent portion of mask 8. An enlarged view including the interface between photoresist film 2 and underlying material 4 is shown in FIG. 2. In FIG. 2, incident exposing radiation photons 10 are represented using open arrowheads. Some of incident photons 10 not absorbed by photoresist film 2 or transmitted to underlying material 4 may be scattered at upper surface 14 of material 4, producing scattered photons 12, indicated using filled arrowheads. It is postulated that increased roughness of surface 14 increases the variation in scattering angles exhibited by scattered photons 12. A large variation in scattering angles is in turn believed to increase the likelihood that scattered photons 12 may penetrate portions of photoresist layer 2 external to the portion subtended by mask feature width Wm. The boundaries of this intended feature width are shown by dashed lines in FIG. 2.
This scattering of exposing radiation into portions of photoresist film 2 external to the intended feature width results in an increased exposed portion of photoresist film 2, as shown in FIG. 3. Exposed portion 16 of photoresist film 2 extends beyond the boundaries of intended feature width Wm, so that the width Wp of the feature transferred to the photoresist is larger than Wm. An error of Wpxe2x88x92Wm is therefore incurred in the transfer of the mask feature to the photoresist. This error may be particularly significant for small intended feature width Wm, since Wpxe2x88x92Wm may become comparable to Wm. For very small intended feature sizes of approximately 0.25 micron or less, root-mean-square (RMS) roughness values on the order of tens of angstroms or less are believed to be capable of producing significant error. Monitoring of roughness values for surfaces of layers to be patterned may therefore be extremely important.
Atomic force microscopy (AFM) is a currently-used technique for determining surface roughness. AFM involves high-resolution scanning (angstrom resolution) of a probe held extremely close to a surface. The chemical force between the probe tip and surface is measured during the scan, producing a high-resolution scan of the surface topography from which RMS roughness may be obtained. AFM can produce accurate measurements of RMS roughness values as low as about 0.5 angstroms, but this accuracy comes at a high cost in measurement time. An AFM scan of an area a few microns on a side may require tens of minutes, or even longer, to perform. To obtain a roughness value characteristic of the overall surface of a semiconductor topography, which is typically formed upon a substrate having a diameter of 8 inches or more, would require sampling multiple areas and could take many hours to obtain by AFM.
In addition to the long measurement times needed for AFM measurement of roughness, sample damage using AFM is also a possibility. Although AFM is theoretically non-destructive, the very close proximity of the measurement tip to the sample allows the possibility of xe2x80x9ctip crashesxe2x80x9d, in which the tip comes into forceful contact with the sample surface during a measurement. Such contact can cause scraping and/or scratching of the surface. In one mode of AFM operation, known as contact mode, the probe tip is in constant contact with the sample surface, which may cause sample damage.
It would therefore be desirable to develop a method for relatively rapid and non-destructive determination of roughness on surfaces of materials used in semiconductor manufacturing. The method should allow determination of roughness over larger areas of a surface than are conveniently scanned using AFM.
The problems outlined above are addressed by a method using a glancing-angle X-ray fluorescence (XRF) technique in combination with calibration standards to determine the roughness of a target surface. In conventional XRF techniques, a beam of primary X-rays is directed at the surface of a semiconductor wafer, and the energies (or corresponding wavelengths) of resultant secondary X-rays emitted by atoms of elements on and just under the surface of the wafer are measured. Atoms of elements in target materials emit secondary X-rays with uniquely characteristic energies. Thus the elemental compositions of materials on and just under the surface of the wafer may be determined from the measured energies of emitted secondary X-rays. In the glancing-angle XRF technique recited herein, a signal containing primary X-rays scattered from the sample surface is analyzed, in addition to a secondary X-ray beam. A plot of the emitted secondary beam strength versus scattered primary beam strength exhibits a characteristic slope. This slope corresponds to a particular value of RMS roughness, which may be determined using a direct imaging or profiling technique such as AFM. After a calibration curve or table relating slope to RMS roughness is generated, subsequent roughness measurements may be performed using only XRF and comparison to the calibration data, without a requirement for further time-consuming AFM measurements.
The glancing-angle XRF measurement may be made using an XRF spectrometer capable of glancing-angle scattering geometries. Such a spectrometer may be obtained commercially. The XRF spectrometer produces a monochromatic primary X-ray beam, which is typically incident upon the sample surface at an angle between about 0.01xc2x0 and about 0.1xc2x0, relative to that surface. Upon interaction with the sample surface, this incident primary beam may be converted into beams which include a scattered primary beam and one or more emitted secondary fluorescence beams. A variation in scattered primary and emitted secondary beam intensities may be produced by altering the experimental configuration, preferably by changing the angle of incidence of the incident beam. Other methods, such as changing the power of the incident beam, may also be suitable for achieving this variation. This variation of the beam intensities allows a set of intensity values to be generated which can subsequently be used to plot secondary beam intensity vs. scattered primary beam intensity. The spectrometer includes an X-ray detector which collects portions of the scattered primary and emitted secondary radiation. The detector is preferably configured directly above the illuminated portion of the surface, in what is known as a total XRF (TXRF) geometry. For each experimental configuration, the output of the detector and associated electronics is typically a plot of intensity versus energy of the collected radiation. The intensities of the scattered primary beam and an emitted secondary beam are recorded for each experimental configuration, typically using a computer associated with the XRF spectrometer.
After collecting data for a series of experimental configurations, intensity of the emitted secondary beam is plotted versus that of the scattered primary beam, and the slope of the plot determined. This slope may be compared to calibration data correlating slope to RMS roughness measured by, for example, AFM. In this manner, RMS roughness of the surface may be determined using a glancing-angle XRF technique. Calibration data may be obtained for each material surface to be studied. It is believed that this is not necessary, however, because the relationship between roughness and slope using the method recited herein has been found to be substantially identical for a range of material surfaces, including silicon, aluminum, titanium, titanium nitride, and silicon dioxide. It is contemplated that this method can measure RMS roughness values which vary in a range from about 1 to about 100 angstroms in topographical elevational disparity. This range of roughness values is relevant to many situations which arise in semiconductor fabrication. For example, an as-deposited polysilicon surface typically exhibits RMS roughness of about 50 angstroms. It is contemplated that this roughness may be reduced using a technique such as chemical-mechanical polishing (CMP). The method recited herein is therefore believed to be useful in monitoring both as-deposited and as-polished polysilicon surfaces. As another example, a tungsten surface which has been planarized by CMP (during, for example, an interconnect formation process) typically exhibits RMS roughness of about 50 angstroms.
In the glancing-angle XRF method recited herein, no mechanical contact is made to the top surface of the sample, so that no surface damage results. No sample preparation is required, and the method is not destructive to the sample. The area on the sample surface from which scattered and/or emitted radiation is detected is typically approximately 2 cm in diameter. Measurement from one such area of the sample, including sample positioning and variation of experimental conditions for data collection, is estimated to require approximately 15 minutes. The measurement area of the method recited herein is therefore believed to be greater, and the measurement time shorter, than those typically achievable using other techniques such as AFM. This increased measurement area and rapidity of measurement may provide greatly increased convenience and throughput for semiconductor process monitoring.